This invention relates to the routing of optical signals in waveguides located in interconnect layers of an integrated circuit (IC)-like structure. More particularly, this invention relates to a new and improved optical waveguide having the capability to route a optical signal vertically between separate horizontal layers of optical waveguides in an IC-like structure. The invention also relates to a new and improved method of fabricating an optical waveguide in an IC-like structure using damascene fabrication process steps that are typically employed in the fabrication of electrical integrated circuits.
The ongoing evolution of microcircuit design has focused on the speed and size of electrical integrated circuit (IC) components, typically in a silicon chip. IC designers have continuously strived to make the IC faster and more functional while taking up less chip space. Currently, interconnection technology is considered as one of several areas that may be advanced to both increase the speed of the IC and to decrease the size of the chip.
For example, only a few years ago spacing between adjoining circuit elements in a typical IC was in the neighborhood of two to three microns. Today, many ICs are being designed at spacing distances as small as 0.35 microns or less. To accommodate narrower spacing and the increased functionality, more layers of conductors are formed above the substrate of the IC, to achieve the necessary number of electrical connections between the more densely located functional elements formed on the substrate of the IC. Advances in fabrication techniques allow as many as five or more separate horizontal layers of interconnect conductors.
Optical waveguides are sometimes considered as replacements or enhancements over the common metal conductors in IC-like structures. Optical signals allow the functional components to operate more quickly or at a higher speed, and unlike electrical signals, optical signals are usually not susceptible to noise and interference. In general, optical signal conduction, with its reduced susceptibility to noise and interference, obtains increased speed in data transmission and processing. Furthermore, due to the coherent nature of laser optical signals and their reduced susceptibility to noise, many more optical signals can be routed in one waveguide or layer of waveguides than is possible using conventional electrical signal interconnect conductors. Therefore, an IC-like structure incorporating optical interconnect waveguides may offer advantages in IC-like structures.
Because of the benefits from optical waveguides, the emphasis on more functionality from smaller sized devices, and the knowledge gained from the development of electrical ICs, it is expected that the evolution of IC-like structures which use waveguides to conduct optical signals will parallel the evolution of electrical ICs. With this historical perspective in mind, IC-like structures which partially or exclusively employ optical waveguides as interconnects are expected to be structured with multiple layers of interconnect waveguides.
The typical optical interconnection is a single waveguide or channel between the two components. In general, the waveguide defines a straight conductive path between conversion devices which convert electrical signals to optical signals and convert optical signals to electrical signals.
A space-effective physical placement and integration of various functional components in an IC-like structure requires considerable flexibility in routing the interconnects. Unfortunately, straight waveguides will not accommodate bends or corners since light signals do not travel around corners. Once a signal propagates the length of the channel, a directional coupler is used to redirect the signal if a change in direction is desired. Directional couplers may substantially increase the manufacturing cost of the IC-like structure, may make effective optical circuit layout impossible or impractical, and may result in a larger IC-like structure.
The previously mentioned patent application relating to the On-Chip Single Layer Horizontal Deflecting Optical Waveguide describes a waveguide and a method of manufacturing it which allow the waveguide to be bent in a horizontal plane. This improvement significantly increases the flexibility and routing of optical waveguides, allowing them to be routed and laid out in an IC-like structure to accommodate greater densities of functional components. However, to fully achieve the densities of functional components in IC-like structures, it will be necessary to also route optical signals from one horizontal layer of optical waveguides into a vertically adjoining horizontal layer of optical waveguides. Similar vertical routing of electrical signals has also been utilized effectively in higher density electrical ICs. However, there is no known previous technique for routing optical signals vertically between horizontal layers of optical waveguide interconnects within the interior of an IC-like structure.
It is with respect to these and other issues that the present invention has evolved.
One aspect of the present invention involves an optical waveguide which routes optical signals vertically between horizontal layers of optical waveguides in an IC-like structure. Another aspect involves forming a vertical optical waveguide between horizontally adjacent layers of optical interconnect waveguides within the interior of an IC-like structure. A further aspect involves allowing optical waveguides to be used as interconnects in an IC-like structure in a manner similar to the horizontal and vertical routing of electrical conductors in an IC. Still another aspect involves splitting an optical signal into multiple optical signals, or combining multiple optical signals into a signal optical signal, in a waveguide which transitions between a horizontal and a vertical orientation in an IC-like structure. Yet another aspect of this invention is to fabricate the present optical waveguide using known and reliable damascene process steps previously used to fabricate electrical ICs.
In accordance with these and other aspects, an optical waveguide extends vertically from a horizontally extending waveguide within an interior of an IC-like structure. The vertical waveguide includes a light reflecting structure positioned at the intersection of the horizontal and vertical waveguides to reflect light between the horizontal and vertical waveguides. A plurality of horizontal waveguides may be vertically separated from one another, with the vertical waveguide extending between the horizontal waveguides. The vertically separated horizontal waveguides may be formed in horizontal planes. A plurality of horizontal waveguides may join the vertical waveguide at a common intersection, to split a single light signal in the vertical waveguide into a plurality of light signals in the horizontal waveguides or to combine a plurality of light signals from the horizontal waveguides into a single light signal. Preferred features of the optical waveguide include forming the reflecting structure as a pyramid-like or a prism-like structure with reflective surfaces on side walls of the structures.
In accordance with other aspects, the invention also involves a method of vertically diverting optical signals from one horizontal waveguide in an IC-like structure into another horizontal waveguide which is vertically separated from the one horizontal waveguide. The method involves forming a via between the two vertically separated horizontal waveguides within the interior of an IC-like structure, and reflecting an optical signal from one horizontal waveguide through the via into the other horizontal waveguide. An optically reflective structure may be placed at an intersection of the horizontal waveguides and the via by which to reflect the optical signals into and out of the via.
In accordance with still other aspects, the invention pertains to a method of fabricating an optical waveguide to connect vertically separated layers of horizontal waveguides in an IC-like structure. The method involves forming a via between two horizontal waveguides in two vertically separated layers of horizontal waveguides, and forming a light reflective structure at an intersection of the via and each horizontal waveguide. The light reflective structure may be formed using high density plasma deposition in conjunction with a plasma sputtering process. The method also preferably relates to depositing dielectric material, forming the via and channels for the horizontal waveguides in the dielectric material, forming the light reflecting structures in the channels, and forming optically transmissive core material in the channels surrounding the light reflecting structures, all accomplished using damascene techniques.
The vertical optical waveguide routes optical signals between horizontal layers of optical waveguides within the interior of the IC-like structure similarly to the manner of routing electrical signals through via plug interconnects between different horizontal layers of electrical conductors in an IC. Consequently, the present invention allows optical interconnects to be employed on an efficient basis in an IC-like structure to facilitate higher densities of functional components and smaller IC-like structures. The IC-like structure may be conveniently formed using conventional damascene fabrication techniques, thereby contributing to the yield productivity in fabricating IC-like devices.
A more complete appreciation of the present invention and its scope, and the manner in which it achieves the above noted improvements, can be obtained by reference to the following detailed description of presently preferred embodiments of the invention taken in connection with the accompanying drawings, which are briefly summarized below, and by reference to the appended claims.